1. Field of the Invention
The present invention relates to a toner for electrophotography which is a stable toner capable of maintaining a suitable tribo electric charge, thereby providing a high image quality so that the formed images do not undergo deterioration even under severe environmental conditions.
2. Discussion of the Related Art
As described in U.S. Pat. Nos. 2,297,691 and 2,357,809 and other publications, the conventional electrophotography comprises the steps of forming an electrostatic latent image by evenly charging a photoconductive insulating layer and subsequently exposing the layer to eliminate the charge on the exposed portion and visualizing the formed image by adhering colored charged fine powder known as a toner to the latent image (a developing process); transferring the obtained visible image to an image-receiving sheet such as a transfer paper (a transfer process); and permanently fixing the transferred image by heating, pressure application or other appropriate means of fixing (a fixing process).
As stated above, a toner must meet the requirements not only in the development process but also in the transfer process and fixing process.
In the above processes, contact heat-fixing methods such as a heat roller fixing and non-contact heat-fixing methods such as an oven fixing can be used for the fixing process. Since the contact method is characterized by having a good thermal efficiency, the temperature of the fixing device can be lowered when compared with that of the non-contact method, thereby making it effective for energy conservation and miniaturization of the overall copying machine. However, in this contact heat-fixing method, the toner is likely to cause a so-called "offset phenomenon," wherein the toner is adhered to the surface of the heat roller, and thus transferred to a subsequent transfer paper. In order to prevent this phenomenon, the surface of a heat roller may be coated with a material excellent in release properties, such as a fluororesin, or a releasing agent such as a silicone oil may be applied on the surface of a heat roller. However, the method of applying a silicone oil necessitates a larger-scale fixing device, which is not only expensive but also complicated, which in turn may undesirably bring about various problems.
Conventionally, vinyl resins typically represented by styrene-acrylic copolymer are used for toners for these systems. When the vinyl resins are used, it is necessary to increase the softening point and the crosslinking density of the resins in order to increase the offset resistance of the toner, which in turn undesirably makes the low-temperature fixing ability of the resulting toner poor. On the contrary, when too much emphasis is placed on the improvement of the low-temperature fixing ability, the resulting toner may suffer from poor offset resistance and blocking resistance. Also, methods in which paraffin waxes, low-molecular weight polyolefins and the like are added as offset inhibitors have been known (see Japanese Patent Laid-Open No. 49-65232, 50-28840 and 50-81342). In these references, however, such problems arise that when the amount of the offset inhibitors added are small, sufficient effects cannot be achieved by the addition thereof, and that when it is large, the deterioration of the obtained developers is undesirably rapid.
On the other hand, as the copying machines are more generalized, they are highly likely to be used under severe environmental conditions, for instance, under high-temperature, high-humidity conditions or under low-temperature, low-humidity conditions. Therefore, formed images which are as clear as those obtained under normal conditions have to be obtained even under such severe environmental conditions. From this aspect, although the vinyl resins provide stable tribo electric charge without deteriorating formed images under the high-temperature, high-humidity conditions, their tribo electric charge undesirably increases under the low-temperature, low-humidity conditions, thereby undesirably decreasing the image concentration of the formed images.
In order to solve these problems, although various proposals have been so far made, the problems have not yet basically been solved.
As for the binding resins used for toners, polyesters which have a wide molecular weight distribution with particularly superior offset resistance and low-temperature fixing ability have been used. The polyesters are resins which are suitably used for providing low-molecular weight components effective for promoting the low-temperature fixing ability. Particularly, in a case where a toner comprises a polyester having an acid value of not less than 5 KOH mg/g, the toner has a good fixing ability at a low temperature and a satisfactorily good offset resistance, so that no deterioration of the developer obtained thereby takes place, even after copying several hundred thousands of sheets. As described above, when the polyester having a relatively high acid value is used for toner production, the resulting toner has an excellent fixing ability and the obtained developer has a long service life and a stable tribo electric charge under low-temperature, low-humidity conditions. However, in certain toner compositions, the tribo electric charge of the developer undesirably increases under high-temperature, high-humidity conditions, thereby showing a lack of environmental stability. On the other hand, in a case where a toner comprises a polyester having an acid value of not more than 5 KOH mg/g, the toner shows excellent properties in that the tribo electric charge thereof does not change even under severe environmental conditions, so that no deterioration takes place in the obtained developer. However, they are not satisfactorily good in the fixing ability.
In order to solve the above problems, the following methods for blending polyester resins having excellent fixing ability with styrene-acrylic resins having a small change in the tribo electric charge under the high-temperature, high-humidity conditions when compared with that under normal temperature, normal humidity conditions have been known. For instance, examples of such methods include:
(1) Methods for blending polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open Nos. 49-6931, 54-114245, 57-70523 and 2-161464);
(2) Methods for chemically binding polyester resins with styrene-acrylic resins (see Japanese Patent Laid-Open No. 56-116043);
(3) Methods for copolymerizing unsaturated polyesters with vinyl monomers (see Japanese Patent Laid-Open Nos. 57-60339, 63-279265, 1-156759 and 2-5073);
(4) Methods for copolymerizing polyester resins having an acryloyl group with vinyl monomers (see Japanese Patent Laid-Open No. 59-45453);
(5) Methods for copolymerizing reactive polyesters with vinyl monomers in the presence of polyester resins (see Japanese Patent Laid-Open No. 2-29664); and
(6) Methods for forming a block copolymer by binding polyester resins and vinyl resins with an ester bond (see Japanese Patent Laid-Open No. 2-881).
However, since the polyester resins have inherently poor compatibility with the styrene-acrylic resins, mere mechanical blending of the components may result in poor dispersion of the resins and the internal additives such as carbon black at the time of production of the toner in certain blending ratios. This may lead to in turn cause a lack of uniformness in the chargeability of the toner, thereby causing such troubles as background in the formed images. Further, when the two types of resins have different molecular weights, the differences in their melt viscosities are likely to take place, thereby making it difficult to make the grain size of the resin for the dispersed domain fine. In such a case, when a toner is produced with such resins, the dispersion of the internal additives such as carbon black becomes extremely poor, so that such problem arises that the resulting toner gives greatly poor image quality. Moreover, in the case where the vinyl monomers are copolymerized with the reactive polyesters, it is applicable only in a restricted compositional range in order not to allow gelation to take place.
Further, toners as binder resins a resin composition having a matrix dispersed with domain particles to provide a microdomain structure have also been known (See Japanese Patent Laid-Open Nos. 4-366176 and 4-366854). However, since the matrices for these resins substantially comprises the styrene-acrylic resins, the problem of fixing failure inherent in the styrene-acrylic resins has not yet been solved.
Further, the present inventors have developed a technique in which the resins obtained by concurrently carrying out the addition polymerization and the condensation polymerization in the same reaction vessel are utilized for the binders for toner production (see Japanese Patent Laid-Open No. 4-142301). Although the binders for toner production disclosed herein have an islands-sea structure formed by dispersing a polyester resin in a styrene-acrylic resin, the diameter of the dispersed particles are larger than 2 .mu.m. Therefore, in this reference, although the fixing temperature can be lowered, a further improvement in a life of the toner cannot be sufficiently obtained.
Accordingly, there is an increasing demand for a binder resin for electrophotography which is excellent in the low-temperature fixing ability and the offset resistance, has an environmental stability in tribo electric charge and image quality, which provides an excellent durability in the resulting toner.